Small electrode spacing electron tube



June 27, 1967 D. e. CONRAD SMALL ELECTRODE SPACING ELECTRON TUBE Filed Dec. 10, 1964 m/VV///////////// m a v m m H I 1/ 1 0 1 Z v I IA 1 N VE N TOR Doe/s 6: Cb/VRAD Aziamgy United States Patent 3,328,625 SMALL ELECTRODE SPACING ELECTRON TUBE Doris G. Conrad, Scotch Plains, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 10, 1964, Ser. No. 417,461 1 Claim. (Cl. 313-294) ABSTRACT OF THE DISCLOSURE In an election tube having a fixed tube geometry with a grid lateral Wire to cathode spacing in the order of 2.5 mils, fixed electrical characteristics, and wherein the grid lateral wire normally has an outer coating of gold, silver, or palladium, means allowing an increase of the grid lateral wire to cathode spacing in the order of 20% without change in the basic tube geometry or change of the electrical characteristics of the tube, comprising substituting a coating of copper for the normal coating on the grid lateral wire. The increased spacing reduces tube manufacturing losses caused by grid to cathode shorts.

This invention relates to electron tubes having small electrode spacings and particularly to small voltage amplifier electron tubes having small grid-to-cathode electrode spacings.

In certain types of electron tubes used for amplifying small voltage signals it is desired that the transconductance of the electron tubes be high in order to provide large voltage amplification. As known, one means for obtaining high transconductance is to position the grid electrode of the electron tubes very close to the cathode electrode. It is not uncommon for the spacing between the grid electrode and the cathode electrode to be three-thousandths of an inch or less. Also, in certain tubes used with very high frequency electrical signals, grid-to-cathode electrode spacings in the order of three-thousandths of an inch or less are also common. One problem associated with the use of such small electrode spacings is that the grid and cathode electrodes have a tendency to short one another. In the mass production of such small electrode spacing electron tubes, it is not uncommon for the loss of production, or shrinkage, to be as high as 20 percent due to gridto-cathode electrode shorting.

Additionally, commercially available electron tubes known as receiving tubes are mass produced in very large numbers, and the electrical characteristics of each tube of a given tube type must fall within rigidly established and published characteristics. Thus, it frequently occurs that known variations in tube construction and processing which might otherwise reduce the loss of production cannot be used in existing receiving type electron tubes since such variations also cause changes in the electrical characteristics of the tubes.

An object of this invention is to provide means for increasing the grid-to-cathode spacing in existing, c0mmercially available electron tubes having small electrode spacings without significantly changing the electrical characteristics of the tubes, thus reducing grid-to-cathode elec trode shorts during manufacture.

For increasing the grid-to-cathode spacing in an existing tube type in accordance with this invention, the electron controlling portions of the grid electrode of the tubes, such as the lateral wires of a conventional wound grid, are provided with an outer surface of copper. It is found that the use of such a copper surfaced grid electrode, in substitution for other material grid electrodes, permits a substantial increase in the grid-to-cathode electrode spacing in the electron tubes in which the substitution is made 3,328,625 Patented June 27, 1967 without causing significant changes in the electrical characteristics of the electron tubes.

The drawing is a transverse section of the electrodes of a pentode type electron tube in which this invention may be used. The electrodes of the tube include a round tubular indirectly heated cathode 10 surounded by a control grid 12 comprising a lateral wire helix 14 wound around and secured to a pair of supporting side rods 16. Additional electrodes include a screen grid 18, a pair of beam plates 20, and an anode 22. As shown, the lateral wire helix 14 is formed to correspond to the shape of the cathode in order to provide uniform control over the emission of electrons from the cathode. The actual construction of the electrodes 10, 12, 18, 20, 22 and the manner in which they are formed and mounted within an electron tube are well known and therefore need not be further described herein.

By way of example, the present invention may be embodied in an existing receiving tube type known as the RCA 6EA8, which is a triode-pentode electron tube generally used as a mixer-oscillator. The pentode of this electron tube has an electrode arrangement similar to that shown in the drawing.

Prior to the present invention, the design value of the grid-to-cathode spacing of such RCA 6EA8 tubes was 2.5 thousandths of an inch, the actual spacing varying within the tolerances of the grid and cathode electrodes. The lateral wire 14 of the control grid 12 was a nickel wire plated with a thin coating of silver. The rated and published transconductance of the pentode of the RCA 6EA8 under operating conditions comprising a bias voltage of 1 volt on the control grid 12, volts on the screen grid 18, 125 volts on the anode 22, and the beam plate 20 connected to the cathode 10, is 6400 micromhos. In the manufacture of such prior art type RCA 6EA8, the shrinkage due to grid-to-cathode electrode shorts often reached values as high as 15 to 20 percent. Attempts to reduce the incidence of grid-to-cathode electrode shorting by increasing the spacing between the grid lateral wires and the cathode were unsuccessful due to the fact that increasing the grid-to-cathode spacing caused a decrease in the transconductance of the electron tubes. Such a decrease of transconductance with increase in electrode spacings is predicted by well known electron tube design theory. Since the RCA =6EA8 is a commercially available receiving tube having published ratings, such a decrease in its transconductance could not be permitted.

RCA 6EA8 tubes are now made with a grid lateral wire 14 plated with copper on a nickel base wire. The spacing between the grid lateral wire 14 and the cathode 10 has a design value of 3.0 thousandths of an inch. That is, the grid-to-cathode spacing has been increased by 20%. The transconductance of the pentode section of the 6EA8, under the aforementioned operating conditions, is still 6400 micromhos. However, shrinkage during manufacture due to grid-to-cathode shorting is now in the order of only 2-4 percent.

Increases in the grid-to-cathode spacing may also be obtained in small electrode spaced tubes having grids of materials other than silver. In general, increased grid-tocathode spacings, without changes in the electrical characteristics, may be obtained by substituting copper for the noble metals gold and paladium used in tubes having gridto-cathode spacings of 4 thousandths of an inch or less.

It is not fully understood why the use of grid electrodes having an outer surface of copper permits an increase of the grid-to-cathode spacing while not causing a corresponding decrease in the transconductance of the electron tube. It is believed, for one thing, that in the widely used method of fabricating grids wherein the lateral wire is wound around a forming mandrel and then slid ofi the mandrel, the copper, being a relatively soft material, flows and acts as a lubricant and decreases the amount of distortion of the lateral wire as it slides off the mandrel. This permits the fabrication of grids that are more accurately dimensioned and more perfectly formed than grids made of other materials. As known, the degree of control by a grid over the electrons emitted from the cathode and the value of transconductance is dependent upon the uniformity of shape and the accuracy of the dimensions of the grid electrode. By using more perfectly formed grids, a higher transconductance from a particular grid'to-cathode spacing may be obtained. Thus, for a given value of transconductance, use of a grid having an outer surface of copper on the electron controlling portions of the grid permits the use of a greater grid-tocathode spacing.

Additionally, it appears that the developed bias, or contact potential, between the grid and cathode electrodes is reduced in the order of 0.2 volt by the use of the copper on the electron controlling portions of the grid electrode. Thus, for a given externally applied bias voltage, the actual bias voltage between the grid and cathode (which is the sum of the externally applied voltage and the grid to cathode developed bias) is less than that when other materials, such as silver, are used, and the electron tube, at a given set of operating voltages, operates at a higher level of plate current and transconductance. Thus, for a given published value of applied bias, the grid-to-cathode electrode spacing may be increased While still providing the electron tubes with the published values of plate current and transconductance corresponding to the given applied bias.

It is believed immaterial whether the electron controlling portions of the grid electrode are made solely of copper or are copper clad or plated. Because of the softness of copper, however, a preferred embodiment in the case of grid electrodes comprising lateral Wires secured to a pair of side rods or a grid frame comprises a coating of copper on a base Wire of a stiffer material such as nickel, molybdenum, or other known grid lateral wire materials.

What is claimed is:

In an electron tube having preselected electrical characteristics and a fixed electrode geometry, and comprising a cathode having an electron emitting surface, a grid electrode having an electron controlling portion adjacent to said cathode surface, and an anode, said electron controlling portion being spaced from the surface of said cathode a distance in the order of 2.5 mils, and said electron controlling portion normally having a surface of gold, silver, or palladium, the improvement characterized by the fact that, without otherwise changing the electrode geometry or electrical characteristics of said tube, the distance between said electron controlling portion and said cathode surface is increased in the order of 20%, and the material of said electron controlling portion is copper.

References Cited UNITED STATES PATENTS 2,460,062 1/ 1949 Charton 313294 2,697,130 12/1954 Korbeiak 313107 X FOREIGN PATENTS 350,938 6/1931 Great Britain.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

